| Modular multilevel converter-based high voltage DC transmission(MMC-HVDC)technology possesses the advantages of flexible control.However,after large-scale offshore wind farms are connected to the grid via MMC,the stability problems of the grid connected system are gradually revealed.The existing research on stability analysis are mostly based on local system modeling and the commonly-used model of MMC is simplified,resulting in unaccurate analysis result the overall instability factors and insignificant effect of the stability improvement.Therefore,this paper focuses on the stability modeling and controller parameter optimization of permanent magnet synchronous generators(PMSG)grid-connected system via MMC,and studies the small-signal modeling,main instability factors analysis and controller parameter optimization method.The main contents of this paper are as follows:(1)In order to study the influencing factors of the various subsystems on the stability of offshore wind farm grid connected system via MMC,considering the dynamic characteristics of the sub-modules of MMC at both ends and the controller link delay,a detailed small signal model of the whole system is developed based on the structural characteristics of PMSG and MMC system.Firstly,the mathematical models of PMSG and MMC considering controller link delay are derived.The 68-order detailed small signal model of grid connected system in d-q rotating coordinate system is then established.Finally,with utilizing PSCAD / EMTDC electromagnetic transient software,a detailed time-domain simulation model of offshore wind farm grid connected system via MMC is established,and the proposed small signal model of grid connected system is compared and verified.(2)Aiming at the issue of quantitatively analyzing the instability factors of the system due to the high order grid connected system,based on participation factor analysis approach,a method for stability evaluation and stability region calculation of grid connected system is proposed.Firstly,the small signal model of the grid connected system is adopted to select the oscillation modes,and the instability factors are analyzed quantitatively based on the participation factor method to obtain the parameters of the system-level leading controller.Then,according to the theory of frequency domain analysis,the calculation method of stability region of grid connected system is proposed to study the influence of variation of leading controller parameters on system stability.Finally,the impact of control link delay and power fluctuation of wind farm on grid connected system stability is investigated by eigenvalue trajectory analysis,which is verified by time-domain simulation.(3)Considering the stable operation area of the dominant controller parameters,an improved particle swarm optimization(PSO)method is proposed to optimize the controller parameters of the grid connected system.Firstly,regarding to the problem that PSO algorithm is easy to fall into local optimum,an improved PSO algorithm is proposed by improving its weight coefficient and learning factor while introducing hybrid mutation mechanism.Then,taking the damping ratio of the oscillation mode and the distance from eigenvalue root to the virtual axis of the grid connected system as the optimization objectives,the objective optimization function of controller parameters is constructed,and the controller parameter optimization method based on improved PSO algorithm is proposed.Finally,the validity of the controller parameter optimization method is verified under three working conditions: the maximum control link delay of two-side converter stations,the power fluctuation of PMSG wind farm under long link delay and the change of dominant controller parameters.The results and conclusions of this paper lay a theoretical foundation and provide technical support for the research on the stability analysis method of the offshore wind farm via MMC grid-connected system and the stability improvement strategy. |
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